1. Field of the Invention
The invention relates to a combustion chamber for a gas turbine and a method of operating a combustion chamber for a gas turbine.
2. Brief Description of the Related Art
Gas turbines operate on the basis of fossil fuel combustion. Fossil fuel combustion processes are these days governed by two major requirements which are in contrast with one another. On the one hand, a combustion process should achieve the highest possible efficiency (so as to save fuel and reduce CO2 emissions); on the other hand, the process should minimise pollutant omissions (for example NOx).
One of the most common ways to improve the efficiency of a combustion process is to use high temperature combustion air preheating. This approach causes combustion to take place at relatively high flame temperatures and eventually the energy of the high temperature combustion gases is transferred to the combustion air using a recuperative or regenerative heat exchanger. One drawback of high preheated air temperatures is that the flame experiences increased peak temperatures, with a disastrous effect upon the thermal-NOx formation path.
Research has been carried out on the combustion of hydrocarbons using diluted reacting mixtures that are kept at a temperature above the self-ignition threshold via the re-circulation of flue gas. The use of the flue gas dilutes the reacting mixture and can be used to provide the energy to allow for self-ignition.
Flue gas re-circulation increases the contents of inerts in a mixture. Early research into the flammability limits for combustion of hydrocarbons and air [Zabetakis, 1965] showed that it is possible to obtain flammable mixtures for re-circulation rates of up to 50%. More recent research aimed at providing reliable operating conditions for practical systems has shown that re-circulation rates of up to 30% can be used as a NOx-reducing technique [Wilkes and Gerhold, 1980]. The flue gas re-circulation rate R is defined as the ratio of the flow rate of the re-circulated flue gas and the flow rate of the fresh mixture fed into the combustion chamber:
  R  =                    G        IR            +              G        ER                    F      +      Ox      
where:
GIR=Flue gas re-circulated inside the combustion chamber;
GER=Flue gas re-circulated outside the combustion chamber;
F=Fuel; and
Ox=fresh oxidant (usually air).
It has however recently been found that it is possible to stabilise a flame at a much higher flue gas re-circulation rate than previously thought. This can produce a mode of combustion that produces a non-visible, non-audible flame. Such a flame is associated with even temperature and concentration profiles, and no hot spots.
This alternate combustion mode, termed for the purposes of this document as “highly diluted combustion”, arises as a result of the very high level of dilution of the reacting mixture. The high level of dilution prevents the formation of localised temperature peaks and thus lowers NOx formation. To achieve an operating set-up that exploits the self-ignition of the flammable diluted mixture, it is necessary to provide a mixture temperature that is above the autoignition threshold. Such a condition will result in a very low temperature difference between the initial and adiabatic flame temperatures, as compared to conventional non-diluted visible flames.
            T      ad        =                            T          in                -                                            Δ              ⁢                                                          ⁢                              H                R                                                    c              p                                ·                      Y            Fuel                              =                        -                                    Δ              ⁢                                                          ⁢                              H                R                                                    c              p                                      ·                  1                      R            +            1                          ·                  F                      F            +            Ox                                          Δ      ⁢                          ⁢      T        =                            T          ad                -                  T          in                    ∝              1                  R          +          1                    
where:
Tad=adiabatic temperature (K);
Tin=initial temperature of the reacting mixture (K);
ΔHR=heat of the reaction (kJ/kg);
cp=specific heat of reacting mixture;
YFuel=molar fraction of burned fuel;
R=re-circulation rate;
F=fuel molar rate; and
Ox=oxidant molar rate.
The above two equations indicate that the difference between the adiabatic temperature (Tad) and the initial temperature (Tin) of the mixture decreases as R increases. The re-circulation rate R acts on the value of the initial temperature (Tin), as this is the result of an energy balance between the re-circulated flue gas and the fresh oxidant stream fed into the combustion chamber. However, the value of R does not affect the value of the adiabatic temperature (Tad), as shown from further elaboration of the above equations in conjunction with standard equations of adiabatic combustion:
            T      ad        =                  T        oxi            -                                    Δ            ⁢                                                  ⁢            H                                c            p                          ·                  φ          ⁡                      (            ϕ            )                                    where    ⁢          :                  φ      ⁡              (        ϕ        )              =                  ϕ        ·                              (                                          Y                Fuel                                            Y                oxi                                      )                    stoich                    +      1      
Toxi=oxidant inlet temperature;
φ=equivalence ratio; and
Yoxi=oxidant mole fraction.
The equivalence ratio parameter (φ) is frequently encountered in the standard literature of combustion, and is simply defined as:
  Φ  =      1    λ  
The relative air to fuel ratio, λ, is defined as:
  λ  =                    (                                  ⁢                              %            ⁢                                                  ⁢            fuel                                %            ⁢                                                  ⁢            air                          )            stoichiometric                      (                                  ⁢                              %            ⁢                                                  ⁢            fuel                                %            ⁢                                                  ⁢            air                          )            actual      
where:
% fuel and % air are the molar percentage (or molar fraction) of fuel and air respectively derived by:
            %      ⁢                          ⁢      fuel        =                  F        Fuel                              F          Air                +                  F          Fuel                                %      ⁢                          ⁢      air        =                  F        Air                              F          Air                +                  F          Fuel                    
and where:
FAir and FFuel are the molar flow rates of air and fuel respectively.
Excess air is defined as: e(%)=(λ−1)*100.
Combustion is usually characterised by the stoichiometry of the reacting mixture.
λ<1(φ>1): fuel rich mixtures—rich stoichiometry
λ=φ=1: stoichiometric conditions
λ>1(φ<1): fuel lean conditions—lean stoichiometry
Conventional gas turbine systems typically operate under lean premixed combustion conditions and employ combustion chambers of the annular, can, can-annular or silos type. Such combustion systems typically rely on a swirl-stabilized flame, in which a small re-circulation zone is formed at the exit of the burners via aerodynamic means. This allows ignition and burnout in a very compact combustor zones, which results in very short residence times (of the order of a few milliseconds) and therefore permits the use of very compact combustion chambers.
Such a system is typically operated with a very lean flame (λ≧2) at around 20 bar, with the oxidant (usually air) preheated to 720 K by compression, and with a flame temperature of around 1750 K. Typical systems have ignition delay times of the order 3 to 5 ms, with residence times of the order of 20-30 ms. Targeted emission levels are: UHC and CO below 10 ppm, and single digit NOx ppm (normalised at 15% O2). These example conditions refer to a gas turbine operating in a full engine load operation mode, and it is necessary to respect the above constraints.
However, such systems are associated with a number of drawbacks. One problem is the generation of self-induced pressure pulsations, which can have dramatic consequences on the mechanical integrity of the combustion system. This problem arises from the small re-circulation zones formed at the exit of each burner. These are not stable and can lead to pressure fluctuations with the combustion chamber termed pulsations.
This tendency for the pressure to fluctuate means that it is necessary to run such systems within constrained operating conditions. Another problem still is the high risk of flashback into the burner, which is an intrinsic characteristic of lean premix systems.
As an alternative to operating with conventional lean premix swirl-stabilized flames, gas turbines could be run in a highly diluted mode, relying on the non-stabilized auto-ignition concept. In highly diluted combustion, a flame ignites spontaneously when enough energy has been released and transferred to the reactants. Thermal energy is then transferred to the reactants as they mix with the re-circulated flue gas. The amount of hot flue gas that needs to be mixed with reactants in order to establish auto-ignition and burnout depends on the rating of the combustion process, and depends on the load for gas turbine systems. The higher the rating (and therefore the process temperatures), the lower the amount of gas dilution needed for auto-ignition and vice versa.
Implementing a highly diluted combustion mode in a gas turbine would allow the flame temperature to be maintained at the desired operating value with a much lower difference between the adiabatic and initial temperatures (ΔT). This would help solve the problem of suppressing high temperature spots, and could bring benefits in terms of emissions levels and combustion efficiency by providing a uniform temperature field.
In order to implement highly diluted combustion is a gas turbine, the system has to operate within the temperature and pressure ranges associated with gas turbines, as well as their characteristic timescales. Gas turbine systems are typically run with the combustion oxidant preheated to 400-500° C. by the compression process. Therefore, no separate recuperators or regenerators are necessary.
However, heat exchangers or alternative heat sources can still be used to further heat the combustion oxidant. As a result of their very lean stoichiometry, gas turbine systems could employ highly diluted combustion with partially premixed or fully premixed flames. Tests performed at atmospheric pressure under operating conditions typical of gas turbine systems have shown that a flue gas re-circulation rate higher than 100% is enough to establish highly diluted combustion, due to the very lean stoichiometry of the system (λ≧2).
In conventionally shaped gas turbine combustion chambers, highly diluted combustion can be established by means of high velocity jets, whose momentum will entrain re-circulated flue gas according to the free jet momentum law.
Chemical kinetics calculations carried out under operating conditions typical of a gas turbine combustion systems have revealed information on a number of gas turbine parameters when operating under highly diluted combustion. It has been found that high flue gas re-circulation rates cause auto-ignition delay times rates that are within the ranges typically required by gas turbine systems (see FIG. 1).
The calculations also have shown that burnout times are not affected by the dilution degree of the mixture. This is because adiabatic or quasi-adiabatic flue gas re-circulation allows for a high enough flame temperature (see FIG. 2).
A beneficial effect of flue gas re-circulation on the NOx formation path has been found. This effect is more pronounced at high pressure, where combustion carried out with a strong flue gas re-circulation rate shows better low-NOx potential than conventional lean premix combustion (see FIG. 3).
It has been further found that flue gas re-circulation rate increases as the load decreases, thus allowing hot gas entrainment at lower operating load and allowing for auto-ignition at lower engine operating regimes (see FIG. 4). FIG. 4 shows the flue gas re-circulation rate calculated via the jet momentum conservation law, whereby:
            M      .                      M        .            0        =                    u        0            u        =                            ρ                      ρ            0                          ·                  A                      A            0                              
For circular jets:
for: x/d0<8
                    K        V            ⁡              (        %        )              =                  [                              0.083            *                          x                              d                0                                              +                      0.0128            *                                          (                                  x                                      d                    0                                                  )                            2                                      ]            ·              ρ                  ρ          0                                for      ⁢              :            ⁢              x        /                  d          0                      >    8                      K        V            ⁡              (        %        )              =          0.32      *              x                  d          0                    *                        ρ                      ρ            0                              
Where:
M=total mass flow rate;
M0=mass flow rate at nozzle exit;
u=mean axial velocity component;
u0=mean axial velocity component at nozzle exit;
ρ=gas density;
ρ0=gas density at nozzle exit;
A=jet cross area;
A0=nozzle area;
x=axial distance from nozzle exit; and
d0=nozzle diameter.
If the flow has a swirling component then the rate of entrainment and the rate of velocity decay are increased. The enhanced entrainment capability of a swirling jet has been defined as:
      K    V    =            (                        0.32          ·                      x                          d              0                                      +                  K          ·          S                    )        ·                  ρ                  ρ          0                    
where:
S=the swirl number;
K=is an empirical constant.
In a conventional gas turbine combustion process, characterized by a swirl stabilised lean premix combustion mode, the overall process time can be defined as:τtot=τtr+τig+τBO 
where:
τtr=the transport time, which in this case is the time necessary to convey the mixture to the stabilization zone;
τig=the ignition delay time;
τBO=the burnout time.
In a highly diluted combustion mode, the characteristic timescales can be defined as:τtot=τtr+τmix+τig+τBO 
where:
τtr=the transport time, which in this case is the time necessary to entrain the hot flue gas;
τmix=the mixing time necessary to mix the oxidant diluted with hot flue gas and the fuel.
In a conventional lean premix system the mixing time is not considered in the equation, as the mixture is considered already perfectly premixed at the exit of the burner. However, as highly diluted combustion is established in conventionally shaped combustion chambers using high velocity jets, such systems are characterized by a longer overall process times as the convective and mixing times are significant. The reactants are entrained (τtr) and mixed with the hot gases (τmix) as the jet develops. When enough hot gas is entrained and mixed to reach the auto-ignition threshold, ignition will occur (τig) and then eventually burnout (τBO).
As a result, highly diluted combustion is associated with the drawback of a longer overall time for the process than for a conventional lean premix system. This results in the need for longer combustion chambers, which is undesirable in gas turbine systems as a result of the augmented mechanical stresses to the shaft.
Another drawback of using conventional gas turbine combustion chambers for highly diluted combustion concerns the mixing process. A very good degree of mixing between air, fuel and the hot gas is a primary requirement for process performance in terms of emissions and thermal failure control.
FIG. 5 is a schematic diagram of a conventionally shaped combustion chamber 200 being used in a highly diluted combustion mode. High velocity jets 250 inject compressed oxidant and fuel into the chamber and flue gas is re-circulated entirely inside the combustion chamber 200. In such an arrangement, the rate of flue gas re-circulation increases with jet velocity (or momentum). However, higher jet velocities are also associated with higher pressure drops. Aerodynamic studies have shown that for a typical gas turbine system the maximum re-circulation rate that can be achieved with simple high velocity jets while respecting the pressure drop constraints varies from 100% to 200%.
In a typical gas turbine system, the maximum pressure drop allowed for the burner module is 3% of the total operating pressure. The use of single free jets could provide re-circulation rates higher than 200%, whilst keeping the pressure drop of the burner/injector module below the 3% limit. However, gas turbines typically operate with very high air to fuel ratios (i.e. very lean mixtures) and severe space constraints, and thus cannot use a burner based on single free jets. The design of high velocity jet injectors is limited by the inherent space constraints associated with gas turbines and the pressure drop limit. In is therefore unavoidable that each jet will interfere with the adjacent jets and the nominal entrapment capability of each single jet will be depleted.
Alternatively, all or part of the flue gas can be re-circulated outside the combustion chamber 200. This can avoid the problem of longer characteristic times, as the re-circulated flue gas could be premixed with the reactants before they enter the combustion chamber. However, such a configuration can result in undesirably high pressure losses which result in a lower process efficiency. Moreover the temperature of the flue gas will be lower due to heat loss during re-circulation outside the combustion chamber. This narrows the operation flexibility of the system and further lengthens the ignition delay and the burnout times.
Therefore on the basis of the above it can be difficult to provide a high enough flue gas re-circulation rate to allow for the onset of highly diluted combustion in a conventionally shaped combustion chamber.